John Snow played a critical role in the development of modern disease surveillance when he:

• A. Devised a more statistically valid method of analyzing epidemiologic data.
• B. Discovered causal agents for anthrax, tuberculosis, and cholera.
• C. Tracked the incidence of tuberculosis in the tenements of New York City.
• D. Used geographic mapping to demonstrate the connection between water supply and cholera.

Answer: (D)

Using geographic mapping to identify disease sources and transmission patterns is a foundational approach in disease surveillance. John Snow demonstrated this by plotting cholera cases on a map of London, revealing a strong cluster around the Broad Street pump. This spatial pattern pointed to the water supply as the source of infection. When the pump handle was removed, cases decreased, providing practical evidence that contaminated water, not an airborne agent, was driving the outbreak. This use of mapping to connect where cases occur with an environmental exposure is what set the stage for modern surveillance and outbreak response, guiding targeted interventions based on location and source.

Other options don’t fit as well because Snow didn’t introduce a new statistical method for analyzing data, didn’t identify causal agents for those diseases, and his famous London cholera investigation focused on water supply rather than tuberculosis in New York tenements.